1. Field of the Invention
The present invention relates to novel lactic acid bacteria belonging to the genus Streptococcus salivarius subsp. thermophilus and variants thereof.
This invention is also concerned with a novel antibacterial substance produced by the above lactic acid bacteria or any one of the above variants.
Further, the present invention pertains to fermented milk starters comprising the above lactic acid bacteria or any one of the above variants and also to processes for producing fermented milk whose acidity increase during storage or transportation is reduced, said processes making use of any one of the fermentation starters.
2. Description of the Related Art
A variety of thermophilic bacteria are found in milk and milk products. They are lactic acid bacteria useful as starters for yogurt or cheese. Among these, some thermophilic bacteria are known to produce a low molecular, antibacterial substance having a molecular weight not higher than 700 [Pulusani, S. R., et al., "Journal of Food Science", 44 575 (1979)]. However, there have not been known yet any thermophilic bacteria which produces a high-molecular, antibacterial substance composed of a protein or peptide or a conjugate thereof as in the present invention.
The term "thermophilic bacteria" as used herein is a generic term for the known bacteria which belong to the genus Streptococcus salivarius subsp. thermophilus. On the other hand, the term "antibacterial substances" as employed herein does not mean the known low-molecular compounds having antibacterial activities such as organic acids--for example, lactic acid--and hydrogen peroxide but is a generic term for substances composed of a peptide, a protein or a conjugate thereof and having antibacterial activities.
In general, antibacterial substances produced by lactic acid bacteria are used to enhance the storability of foods or to avoid deterioration in quality of foods due to their contamination by saprophytes, as typified by nisin produced by certain Lactococcus lactis subsp. lactis (formerly, Streptococcus lactis subsp. lactis).
In the case of a fermented food containing viable cells therein, it is extremely difficult to maintain its favorable taste and flavor, which the fermented food has immediately after its production, continuously during its storage and distribution period. For example, yogurt which features the inclusion of viable cells therein is accompanied by the taste- and flavor-related drawback that its acidity increases during its storage and distribution period and its sour taste becomes stronger. A variety of methods have heretofore been attempted with a view toward preventing an acidity increase during storage of yogurt. These methods were however all impractical because they were unable to bring about sufficient effects, rendered the production process extremely complex and cumbersome, or resulted in a very high production cost.
Fermented foods such as yogurt contain viable cells therein as described above. These viable cells remain active during storage so that they may deteriorate the taste and flavor of such fermented foods during their storage.
To avoid deterioration in taste and flavor of fermented foods by such a cause, it is most preferable to inhibit the activity of the bacteria, said activity being a cause for the deterioration in taste and flavor, without modifying their conventional production steps.
Yogurt contains Lactobacillus delbrueckii subsp. bulgaricus as principal starter bacteria together with one or more thermophilic bacteria. The former bacteria are a primary cause for acidity increase at least during the storage and distribution period. It is hence possible to effectively avoid deterioration in quality due to acidity increase during the storage and distribution period if the growth of the former bacteria can be suppressed and the production of the acid by the bacteria strain can be minimized directly.